Page 586 & 587
Actin-binding proteins control the behavior of actin filaments in vertebrate cells Page 587
* Organization of actin filaments: Individual actin filaments are assembled into two general type of structure, called actin bundles and actin networks . e.g.microvilli Page 587
Actin Bundles Page 587
A Cortex Rich in Actin Filaments Underlies the Plasma Membrane of Most Eukaryotic Cells Page 588
A Cortex Rich in Actin Filaments Underlies the Plasma Membrane of Most Eukaryotic Cells Page 588
Cell Crawling Depends on Cortical Actin Page 588 & 589
Page 588 & 589
Forces generated in the actin-filament-rich cortex help move a cell forward Page 588 & 589
Cell Migration via Lamellipodia Involves Cycles of Protrusion, Attachment,Translocation, and Detachment The integrin-dependent attachments are called focal contacts
A web of polymerizing actin filaments pushes the leading edge of a lamellipodium forward Page 590
Page 591
All actin-dependent motor proteins belong to the myosin family Page 591
Page 591
Activation of Rho family GTPases can have a dramatic effect on the organization of actin filaments in fibroblasts Page 592
Signal-Induced Changes in Actin Cytoskeleton
Signal-Induced Changes in Actin Cytoskeleton Lysophosphatidic acid Page 592
(e) Regulation of Rho family proteins. Guanine-nucleotide exchanges factors (GEFs) stimulate exchange of a bound GDP for GTP, activating Rho family proteins, and allowing them to stimulate actin remodeling. GTPase activating proteins (GAPs) stimulate Rho GTPases to hydrolyze their bound GTP, thereby inactivating them. Guanine-nucleotide displacement inhibitors (GDIs) can sequester Page 592
  You are examining a cell line in which activation of the Rho family member Rac promotes lamellipodia formation. The following statements is true or False , please explain it. (a) Cells carrying a Rac mutation that makes Rac act as if it is always bound to GTP will polymerize more unbranched actin filaments than normal cells. (b) Cells carrying a Rac mutation that makes Rac unable to exchange GDP for GTP will polymerize more unbranched actin filaments than normal cells. (c) Cells carrying a Rac mutation that makes Rac unable to exchange GDP for GTP will polymerize more branched actin filaments than normal cells.
Three kinds of vertebrate muscle Page 592
Page 593 & 594
(muscle cell) 肌 內 膜 肌 束 膜 肌 外 膜 肌 束 Myofibril Structure of Skeletal Muscle Page 594
Muscle Fibers (Muscle Cells) Page 594
A skeletal muscle cell is packed with myofibrils Each myofibril is subdivided along its length into repeating units called sarcomeres. Page 594
Sarcomeres are the contractile units of muscle Page 594
All actin-dependent motor proteins belong to the myosin family
Myosin II Page 594
Page 594
Sarcomeres are the contractile units of muscle Page 594
Muscles contract by a sliding-filament mechanism Page 594 & 595
Sliding Filament Theory Page 594 & 595
The head of a myosin-II molecule walks along an actin filament through an ATP-dependent cycle of conformational changes Page 594-596
The Cyclic Process of Muscle Contraction rigor mortis Page 594-596
Sarcoplasmic Reticulum (SR) Page 595-597
Page 595-598
Myofilament Structure
Myofilament Structure Page 597
The Regulation of Muscle Contraction Depends on Calcium Page 597
• Tropomyosin physically blocks cross bridges. • Troponin complex: – Troponin I inhibits binding of myosin. – Troponin T binds to tropomyosin. – Troponin C binds to calcium. Page 597 & 598 Skeletal Muscle Contraction Is Controlled by Tropomyosin and Troponin Complexes
Page 598
Cardiac Muscle
Cardiac Muscle Page 598
Calcium Channels
Calcium Channels • Unlike skeletal muscle, the voltage-gated calcium channels are not directly connected to calcium channels in the SR. – Instead, calcium acts as a second messenger to open SR channels. – Called calcium-induced calcium release – Excitation-contraction coupling is slower. Page 598
Role of Calcium in Cardiac Muscle
Role of Calcium in Cardiac Muscle Page 598
Page 598
Smooth Muscle
Smooth Muscle • Found in blood vessel walls, bronchioles, digestive organs, urinary and reproductive tracts – Produce peristaltic waves to propel contents of these organs Page 598
Smooth Muscle
Smooth Muscle • No sarcomeres • Long actin filaments attached to dense bodies • Some myosin filaments • Arrangement allows contraction even when greatly stretched Page 598
Page 598
Page 598
Regulation of Contraction in Smooth Muscle Cells Page 598
Excitation-Contraction Coupling in Smooth Muscle
Excitation-Contraction Coupling in Smooth Muscle Page 598
Excitation-Contraction Coupling in Smooth Muscle
Excitation-Contraction Coupling in Smooth Muscle • Begins with rise in intracellular calcium concentrations – Only some comes from SR. – Most comes across plasma membrane after voltage-gated calcium channels are opened. • Calcium binds to calmodulin (no troponin in smooth muscle). – Activates myosin light chain kinase Page 598
Excitation-Contraction Coupling in Smooth Muscle
• Myosin light chain kinase phosphorylates myosin light chains. – This allows myosin to form cross bridges with actin to initiate contraction. • Stimulation is graded. More stimulation allows in more calcium, which allows stronger contractions. • Contractions are slow and sustained. – May enter a “latch state” Excitation-Contraction Coupling in Smooth Muscle Page 598
Smooth Muscle Relaxation
Smooth Muscle Relaxation • Calcium is pumped out using calcium ATPase active transport pumps. • Calmodulin dissociates from myosin light chain kinase. • Phosphate groups are stripped from the myosin by myosin phosphatase . Page 598
What are mechanisms of cardiac muscle and smooth muscle contraction?
True or Flase, please explain it. (a) All eukaryotic cells have actin, microtubules, and intermediate filaments in their cytoplasm. (b) The cytoskeleton provides a rigid and unchangeable structure important for the shape of the cell. (c) The three cytoskeletal filaments perform distinct tasks in the cell and act completely independently of one another. (d) Actin filaments and microtubules have an inherent polarity, with a plus end that grows more quickly than the minus end.